This patent specification relates to the field of ultrasound information processing systems. In particular, it relates to a method and system for generating ultrasound frames having decorrelated speckle patterns and generating compound ultrasound images therefrom.
Ultrasound imaging systems have become increasingly popular for use in medical diagnosis because they are non-invasive, easy to use, capable of real-time operation, and do not subject patients to the dangers of electromagnetic radiation. Instead of electromagnetic radiation, an ultrasound imaging system transmits sound waves of very high frequency (e.g., 1 MHz to 15 MHz) into the patient and processes echoes scattered from structures in the patient""s body to derive and display information relating to these strictures.
One factor that currently limits the output quality of ultrasound imaging systems is the phenomenon of speckle. Speckle arises from the use of coherent signals to acoustically interrogate a target. Complex interference patterns arise from phase variations in the coherent signals as they propagate through, and reflect from, the many large and small acoustic reflectivity boundaries in the target. These phase variations may be caused by diffuse scatterers, by multiple scattering, by a non-homogeneous propagation medium which distorts the phase of the received wave, or by other factors.
Speckle appears to the viewer like random noise superimposed on the output image, and degrades the contrast resolution of the image (i.e., the accurate portrayal of the acoustic reflectivity of respective target locations). A speckle pattern will change in a visually recognizable (although complex) way upon a small displacement or rotation of the ultrasound transducer relative to the target, or upon small movements of the target tissue.
One common approach to reducing speckle involves image compounding, i.e., the combining of multiple component frames into an output image, the component frames having decorrelated, or at least partially decorrelated, speckle patterns. Most generally, decorrelation of the speckle patterns relates to how similar they are, or the degree to which their grainy structures appear to be derived from one another. As applied to image compounding, decorrelation of the speckle patterns relates to the degree to which compounding would reduce the speckle effects. Thus, for example, the compounding of two entirely correlated speckle patterns would cause little reduction in the amount of speckle. However, as the decorrelation of the speckle patterns is increased, compounding the patterns would result in speckle reduction, up to a maximum value when the two patterns became entirely decorrelated. It can be shown that, where direct frame averaging is used, this maximum value for speckle reduction is 2, or more generally N for N component frames. Mathematically, decorrelation of the two speckle patterns can be expressed by a measure such as a correlation coefficient, wherein a correlation coefficient of 1.0 corresponds to entirely correlated speckle patterns and a correlation coefficient of 0.0 corresponds to entirely decorrelated, speckle patterns.
Spatial compounding refers to the compounding of frame data from different sub-apertures and/or angular viewpoints for a given target location. Examples of spatial compounding from different angular viewpoints (xe2x80x9clook anglesxe2x80x9d) can be found in U.S. Pat. No. 6,117,081 (Jago et. al.), U.S. Pat. No. 6,126,598 (Entrekin et. al.), U.S. Pat. No. 6,126,599 (Jago et. al.), and U.S. Pat. No. 6,135,956 (Schmiesing et. al.), which are incorporated by reference herein. There is a trade-off between speckle reduction and spatial resolution in such systems. For example, when using different sub-apertures, it is generally required that the relative translation of the sub-apertures be more than one-half the size of the sub-apertures to yield decorrelated speckle patterns. However, if more sub-apertures were formed to achieve this spacing, there would be a corresponding decrease in the spatial resolution of each component frame as the size of each aperture is decreased. Alternatively, if panoramic or extended view imaging is used to achieve a speckle reduction effect, image registration errors substantially reduce the spatial resolution.
Similar disadvantages are incurred in angular compounding systems such as those listed above, in which frames are taken from different xe2x80x9clook anglesxe2x80x9d and compounded. As described therein, it is required that the xe2x80x9clook anglesxe2x80x9d of the component frames be at least several degrees apart to achieve sufficiently decorrelated speckle patterns. However, as the angular separation of the xe2x80x9clook anglesxe2x80x9d increases, there are beam steering and registration errors that reduce spatial resolution, as well as effective aperture reductions that reduce spatial resolution. Moreover, these errors get worse as the angles deviate further from the normal to the transducer, because small beam steering errors take on increased significance at these angles. Finally, these angular compounding systems suffer from grating lobes due to aliasing effects.
Frequency compounding is accomplished by dividing the bandwidth of the imaging system into multiple bands, and then processing and compounding signals from the different frequency bands. There is a trade-off between axial resolution and speckle reduction in these systems. For increased speckle reduction, it is desirable that the frequency bands of the interrogating pulses have lesser overlap in the frequency domain. However, to achieve this lesser overlap, the bandwidth of the interrogating pulses needs to be narrower, which corresponds to increased pulse length in the time domain and therefore reduced axial resolution. Frequency compounding also causes lateral resolution degradation due to contributions from the lower frequency component, thereby further decreasing spatial resolution.
Temporal compounding involves averaging successive frames together into a compound image. Because only one acoustic pulse can be sent into the target at a time, the above spatial compounding and frequency compounding techniques inherently involve temporal compounding as well. In theory, xe2x80x9cpurexe2x80x9d temporal compoundingxe2x80x94in which no locations, angles, or frequencies are changed between framesxe2x80x94may not reduce speckle at all because the speckle pattern should not change between frames. In practice, however, many tissues and scattering structures incur a small amount of movement between component frames (e.g., through respiratory movements, gastric movements, small muscle movements, etc.) such that speckle patterns can change continually between component frames. Because no transducer movement, angle changes, or frequency changes are incurred between component frames, xe2x80x9cpurexe2x80x9d temporal compounding involves little or no loss of spatial resolution.
However, spatial compounding, frequency compounding, and temporal compounding each involve an additional trade-off between speckle reduction and temporal resolution, i e., the ability to xe2x80x9ckeep upxe2x80x9d with moving tissue and/or a moving transducer. As more frames xe2x80x9cNxe2x80x9d are compounded to reduce speckle, the output image becomes increasingly blurry for locations of relative movement between the transducer and the target tissue, and/or the output frame rate is decreased.
Proposals have been made for dealing with the undesirable tradeoffs between speckle reduction and spatial and/or temporal resolution. For example, the ""598 patent supra proposes a dynamic trade off between the blurring effect and the speckle effect, wherein the number of spatially compounded frames xe2x80x9cNxe2x80x9d is automatically reduced during fast tissue or transducer motion. The ""081 patent supra proposes a substitute tradeoff, one between the blurring effect and the frame rate, wherein the xe2x80x9cNxe2x80x9d frames being compounded are first corrected for misregistration prior to compounding, albeit causing a concomitant reduction in output frame rate, and calling for a substantial increase in processing power and system complexity.
However, it is believed that these and other such proposals can be seen as representing xe2x80x9cpatchesxe2x80x9d for fundamental shortcomings in the current ways that speckle is dealt with. In accordance with the preferred embodiments, it is believed that speckle effects can be more effectively reduced by systematically dealing with the many parameters that affect the speckle patterns themselves.
Accordingly, it would be desirable to provide an ultrasound imaging system that provides for reduced speckle while reducing the degradation in spatial resolution associated with image compounding.
It would be further desirable to provide an ultrasound imaging system that provides for reduced speckle while reducing the degradation in temporal resolution associated with image compounding.
It would be further desirable to provide an ultrasound imaging system that provides for reduced speckle that does not substantially increase the complexity of conventional image-compounding ultrasound systems.
In accordance with a preferred embodiment, an ultrasound system that generates compound images from component frames having decorrelated speckle patterns is provided. Successive sets of distinct, speckle-affecting parameters are used to generate the successive component frames for compounding, and are selected such that the successive component frames have decorrelated speckle patterns. The speckle-affecting parameters that are changed from frame to frame can be selected from a wide variety of parameters, including transmit beamformer parameters, receive beamformer parameters, and demodulator parameters. The speckle-affecting parameters that are changed from frame to frame include, but are not limited to, transmit frequency, number of transmit cycles, number of transmit zones, transmit pulse shape, transmit focus profile, transmit focal point, transmit steering angle, transmit F-number, transmit focus algorithm identifier, transmit aperture setting, transmit apodization profile, receive bandwidth, receive focus profile, receive focal point, receive steering angle, receive F-number, receive focus algorithm identifier, receive aperture setting, receive apodization profile, local oscillator frequency, and receive pulse shape.
According to a preferred embodiment, the successive sets of speckle-affecting parameters differ from each other by at least two speckle-affecting parameters for enhancing the decorrelation of the successive component frames, thereby providing for improved contrast resolution in the compounded image. According to another preferred embodiment, the amount by which each of the multiple speckle-affecting parameters is changed is less than a decorrelation threshold for that parameter. A decorrelation threshold relates to an amount that a speckle-affecting parameter alone would be required to change in order to yield decorrelated speckle patterns, if no other parameters were changed. It has been found that when more speckle-affecting parameters are changed, each speckle-affecting parameter can be changed by an amount less than its decorrelation threshold, and yet decorrelated speckle patterns can still be obtained. Moreover, because two different types of speckle-affecting parameters tend to alter the component frames in different ways, it has been found that the spatial resolution of the compounded image can be better as compared to the scenario in which only one speckle-affecting parameter is altered by its decorrelation threshold.
In one example of the above preferred embodiment, a partial frequency compounding method and system is provided. According to a preferred method, the difference in transmit frequency between successive frames is only a partial fraction of the difference that would otherwise be required to establish speckle pattern decorrelation if only transmit frequency, and no other parameter, were changed. However, according to a preferred method, an additional speckle-affecting parameter is also changed between successive frames by an amount sufficient to yield decorrelated speckle patterns when changed in conjunction with the transmit frequency. Thus, as a conceptual example, if the transmit frequency would alone need to be changed from f0 to 1.5f0 to establish decorrelated speckle patterns, then according to the preferred embodiments, the transmit frequency could be only changed from f0 to 1.2f0 as long as the number of transmit cycles, for example, were changed from n0 to 1.5n0 at the same time. Moreover, because the resulting component frames are changed in different ways by the transmit frequency change versus the transmit cycle change, the spatial resolution of the compounded result can be better than if only the transmit frequency were changed from f0 to 1.5f0 between component frames.